a) Field of the Invention
This invention relates to a fuel injection control system and method suitable for use in an internal combustion engine of the system that fuel is injected into an intake pipe.
b) Description of the Related Art
Intake pipe fuel injection control systems have found wide-spread commercial utility in recent years, because they can more readily perform high-accuracy control on the quantity of fuel to be fed, maintain an adequate air/fuel ratio and also meet the move toward internal combustion engines (which may hereinafter be called merely the "engines") of higher power output.
An internal combustion engine equipped with such a fuel injection control system enjoys such merits as described above but, on the other hand, involves the problem of transitional fluctuations in air/fuel ratio due to the existence of adhered fuel in an intake pipe.
Described specifically, fuel is not injected directly into a cylinder but is injected into the injection pipe, so that a portion of the fuel so injected adheres an inner wall of the intake pipe and an evaporated portion of the adhered fuel is then fed into the cylinder. Even if fuel is injected in a quantity corresponding to the quantity of inducted air, fuel may be fed too little or too much into the cylinder in a transition period upon acceleration, deceleration or the like, leading to a potential problem of inducing a misfire, fluctuations in air/fuel ratio, a deterioration to exhaust gas, or the like.
To cope with such a potential problem, techniques have been provided for performing control upon acceleration or deceleration by calculating the quantity of adhered fuel and correcting the quantity of fuel to be injected.
According to the technique disclosed, for example, in Japanese Patent Application Laid-Open (Kokai) No. HEI 4-36032, a fuel injection quantity Gf is controlled by calculating it in accordance with the following formula: EQU Gf={[Qp/(A/F)]-.beta..beta..multidot.Mf(n)}/(1-XX) (1) EQU Mf(n)=(1-.beta..beta.).multidot.Mf(n)+XX.multidot.Gf (2)
where
Qp: the quantity of inducted air, PA1 A/F: the target air/fuel ratio, PA1 Mf(n): the quantity of fuel remaining one cycle before in an intake port in an n-cylinder engine, PA1 .beta..beta.: the rate of evaporation of fuel in the intake port between an intake stroke and the next intake stroke in a cylinder, and PA1 XX the rate of adhesion of injected fuel on an inner wall of the intake port. PA1 TTRNS (n): the predicted feed quantity, PA1 TB(n): the basic injection quantity, PA1 TTRNSX(n'): the first-part feed quantity in the same cylinder immediately before the present injection, PA1 TTRNSY(n'): the second-part feed quantity in the same cylinder immediately before the present injection, and PA1 .alpha.: the direct feed rate. PA1 TTRNSX(n): the present first-part feed quantity, PA1 TTRNSY(n): the present second-part feed quantity, PA1 TINJ(n): the actual injection quantity in which fuel has been injected immediately before the present injection, PA1 TTRNSX(n'): the first-part feed quantity in the same cylinder immediately before the present injection, PA1 TTRNSY(n'): the second-part feed quantity in the same cylinder immediately before the present injection, PA1 X: the first smoothing factor, PA1 Y: the second smoothing factor, PA1 .alpha.: the direct feed rate, and PA1 .beta.: the distribution coefficient.
Further, means for performing the above calculation is constructed based on the concept that the quantity of evaporation of adhered fuel is a first-order delay response and the sum of the quantity of evaporation of the adhered fuel and the quantity of fuel directly fed without adhesion is the feed quantity of fuel.
Control by such conventional calculation means is however accompanied by problems to be described next.
Adhered fuel includes not only fuel adhered on an inner wall of an intake pipe but also that adhered on an intake valve. The intake valve becomes as hot as about 200.degree. C. during operation so that the temperature of the intake valve is higher than the temperature of the inner wall of the intake pipe, the latter temperature being about 80.degree. C. or so. Accordingly the fuel adhered on the intake valve is prone to evaporation and has a higher velocity of evaporation.
Further, the inner wall of the intake pipe and the intake valve are different from each other in the characteristics of a temperature increase responsive to the state of operation of the engine.
The rate of evaporation of fuel cannot be expressed by a single characteristic value like .beta..beta. in the formula described above. This also indicates that a fuel feeding system is not characterized by such a simple first-order delay characteristic as has been recognized generally. The rate of evaporation of fuel exhibits substantial influence especially in a transition state of operation. To perform good control even during such a transition state, it is necessary to effect a correction with the above-described evaporation characteristics in view.
Of the injected fuel, the fuel to be fed directly into the cylinder includes that to be fed as a result of prompt evaporation subsequent to its adhesion on the inner wall of the intake pipe and the intake valve. When the velocity of evaporation drops in a cold state, the rate of fuel to be fed directly becomes smaller so that conventional control means cannot perform appropriate control on the quantity of fuel to be injected. Hence a correction is also needed in this respect.